How does this breather hose work?

My 2003 SVT has been running like E36 M3e for months with crummy gas mileage. I could tell it had a massive vacuum leak but didn't have 30 seconds to peak under the hood until this week. Luckily the issue was obvious with a broken connector from the breather hose between the valve cover and the air cleaner assembly. I temporarily plugged the breather hose with a wine bottle stopper (fit perfectly!)

The valve cover has a rubber tube that connects to the air cleaner and it filters through a tiny sponge like cube into the air cleaner. I am assuming this is part of the PCV system. What I don't get is how when this the hose is disconnected in the wild there is a massive vacuum leak. But if I connect it to the air cleaner it is still basically open but inside the air cleaner housing, so why is it not a vacuum leak.?

My theory is that the intake in the air cleaner is a massive volume suck of air that essentially creates a vacuum on the breather hose. Is there some sort of Venturi effect that pulls blow by oil out of valve cover back into the combustion chamber? Yeah random trivial subject. 

The pistons moving down create a low pressure when the intake valve is open.  This makes the higher pressure atmospheric air run into the cylinder.  This is the vacuum.  It, in theory, will only really pull vacuum under open throttle loads.  

This is good because that's when you want to have a vent on the crankcase AND the engine can deal with ingesting that small bit of oily residue without concern.

It's not a vacuum leak because the air will be accounted for by the maf.

On the PCV system, it's a closed loop system- what you are dealing with is the clean air side.  There's another connection someplace else where the crank air is directed into the intake manifold.  Which means that *most* of the air that passes through the crank is metered by the air meter (it's measured before it goes into the engine and comes back out again).  And a system like this constantly moves air to keep it reasonably clean.

So the leak comes from the fact that all of the air drawn by the manifold side isn't being measured.

Intake vacuum is hooked to the crankcase to pull blowby fumes from the crankcase and send them through the incinerator (combustion chambers) before releasing them to atmosphere. If the crankcase was otherwise sealed it would be pulled to  full 20" intake vacuum and this would cause other issues, so it has an air inlet that comes from the intake system. Usually that air passes across the mass air flow sensor before it goes into the crankcase. If the PCV system is ingesting air that hasn't gone through the MAF and been accounted for, there won't be fuel to match it and the car will run lean. 

This is very closely related to why many turbo cars will have a stumble after letting off the gas if you divert the stock blow off valve to blow to open atmosphere instead of blowing back into the intake tract. "Unmetered air" = wrong air fuel ratio.

In reply to alfadriver :

meter being the PCV valve..